alan.melia at btinternet.com
Sat Aug 14 23:47:46 UTC 2010
Surely all this is a case of an engineer being able to
Manufactures specify to sell parts, most of the important parameter are
minimum values, and the range is as wide as it can be. If you want a
parameter "bracketed" then you must specify that when buying and pay the
premium or take the expense of testing incomming batches for sensitive
parameters. I used to give lectures to new engineers on reading
specifications .....and what "Absolute maximum" actually means. Clever
engineers also love using operations that rely on unspecified parameters.
For example using a stock switching diode as a spike quencher in a relay
driver. The usually work (but not always!) but are generally not specified
for that service.
Spice parameters are probably a few "typical" samples of even "the one I
happened to choose to measure"!!
I had a case of some +/-12v transistor logic which made it was into a big
telecoms project. the prototypes worked fine. By the time of bulk
manufacture the supplier of the the main transistor had had several cost
improvement stages. The result was the kit was unreliable. The problem was
that first the base junction were swung to -12v by the "0" logic state and
second from a transistor with a fairly simple round dot geometry emitter,
the device had been replaced by an interdigital high ft chip (it still met
the basic "greater than" specs.)
The interdigital device suffered severe loss of gain after a period of
avalanching at several millamps (I think it also suffered electromigration)
much more than the circular geometry transistors. I think this was due to
the increased emitter periphery length. It was solved by placing a simple
diode in the emitter leg, or a clamp on the base. The engineer had not read,
or not understood the meaning of the Vebmax = 5v parameter, and his
prototype had worked. The production side were not very happy but eventually
were forced to junk thousands of 8inch square pcbs and do a re-layout. This
was in the days before computer simulation !! It might have helped!
----- Original Message -----
From: "jimlux" <jimlux at earthlink.net>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Saturday, August 14, 2010 10:42 PM
Subject: Re: [time-nuts] Simulation
> Hal Murray wrote:
> >> Line got moved to other side of big ocean. Process got " tweaked" beta
> >> now 4x what it was.
> > I'm pretty sure that military grade parts have paperwork and processes
> > cover that case. I expect it costs a lot.
> > I think the same sort of service is available to high volume customers
> > less than military price.
> just so..
> In the space business, we call it "traceability to sand"... you haven't
> lived til someone has a failed 2n2222, somwhere on some piece of
> critical hardware, and they issue a GIDEP alert, and then the mission
> assurance folks call you up and ask, "you don't by any chance have
> 2N2222's in your flight hardware do you?".. then there's the whole
> manufacturer and date code hunt.. Looking through the build
> documentation to find out. (or worse yet, if you had decided for some
> reason to use the prototype, which you didn't keep such good records on,
> but which you have photos of, and trying to read the date codes off the
> assembled item with a magnifying glass)
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